(1) Field of the Invention
The present invention relates to a technique for supplying toner from a toner cartridge to a development unit in an image forming apparatus, and in particular to a technique for detecting the amount of toner that remains in a toner buffer provided between the toner cartridge and the developing unit.
(2) Description of the Related Art
In image forming apparatuses, a buffer is provided between a toner cartridge and a developing unit for feeding toner from the toner cartridge to the developing unit. Such a buffer stores therein a certain amount of toner and feeds the stored toner to the developing unit in response to a request from the developing unit. The presence of the buffer is tied to the following two purposes. One is to control the amount of toner that is to be fed to the developing unit. The other is to prevent a situation where a toner shortage occurs and the image forming apparatus cannot perform any operation.
In order to supply toner from the toner cartridge to the buffer, the amount of toner that remains in the buffer is detected first, and if it is detected that only a small amount of toner remains, a predetermined amount of toner is supplied from the toner cartridge to the buffer.
As a method for detecting the amount of toner that remains in the buffer, a method using a piezo sensor and a method using an optical sensor are known. The piezo sensor is used for measuring fluctuation of a liquid level of the toner stored in the buffer, and the optical sensor is used for measuring light transparency in the buffer. Japanese Patent No. 4200612 discloses a detection method using the optical sensor.
Meanwhile, it is required for a black-and-white image forming apparatus to be produced at a lower cost than a full-color image forming apparatus. For the purpose of reducing the production cost, some black-and-white image forming apparatuses are provided with a magnetic reed switch that is more economical than the optical sensor. The magnetic reed switch is turned ON and OFF in response to a change in magnetic field (i.e. magnetic variation) occurred in a sensing range.
Specifically, a toner level indicator plate is swingably disposed within the buffer to swing up and down about a fixed point in response to a change in a liquid level of the toner that remains in the buffer. A magnet is attached to a free edge of the toner level indicator plate. The magnetic reed switch is positioned at such a level that the magnet is made to face the magnetic reed switch in a state where the detection plate has swung down. As the liquid level of the toner is lowered, the detection plate along with the magnet moves down. At this time, the magnet brings about the change in the magnetic field. In response to the change in the magnetic field, the signal output from the magnetic reed switch is switched. As a result, it is detected that the amount of toner that remains in the buffer is small.
Recently, due to a need to reduce production costs, there is an increasing demand for using such a reasonable magnetic reed switch for the detection of the amount of toner in the full-color image forming apparatus as well.
Inside a full-color image forming apparatus, a toner cartridge, a buffer, a developing unit, and others are disposed for each of yellow (Y), magenta (M), cyan (C) and black (K). Compared with a black-and-white image forming apparatus, the full-color image forming apparatus has less layout flexibility for the above-mentioned components that are situated closely to each other. In such a structure, if a magnetic reed switch is used for detecting the amount of toner that remains in a buffer, it may be inevitable to situate the motor close to the magnetic reed switch.
However, situating the motor close to the magnetic reed switch poses a problem that the magnetic reed switch is affected by a change in magnet field caused by a rotational movement of the motor and sequentially outputs a signal indicating ON/OFF. As a result, the image forming apparatus cannot appropriately detect the amount of toner that remains in the buffer. This hinders appropriate control of toner supply from the toner cartridge to the toner buffer.